 One of the objectives from our lesson is to understand the role of the changing sun angle and the tilt of the earth on its axis and the roles that those play in the changing of the seasons. And so to look at that we're going to start with a virtual flashlight. And we have a situation here where our virtual flashlight is pointed directly at an object so the light comes in perpendicularly and strikes the object. You can see that the light is fairly bright, it's spread over a small area so the amount of light per unit area is high and the light is intense as it strikes the object. But if we change the angle of our flashlight and make the light come in at a lower angle and strike the surface, the light now gets spread out over a larger area so the amount of light per unit area is now lower and the light is dimmer on the surface. It's not as intense so even though the same amount of light is coming out of the flashlight it's dimmer on the surface because of the lower angle that is striking the surface. You could try that experiment at home with a real flashlight if you wanted but in a nutshell you've seen the reason for the change of seasons. Earth is tilted on its axis at a 23.5 degree angle and as it revolves around the sun the location that gets the direct beam of light when it's coming in perpendicularly, that location changes throughout the year. It's always between 23.5 degrees north latitude and 23.5 degrees south latitude. For example over the equator the equator gets the direct beam of light twice a year on the spring equinox and the autumnal equinox and during other parts of the year that direct beam is over other various locations between 23.5 north and 23.5 south. To better see how the changing sun angle impacts the seasons we have an animation here of the earth revolving around the sun and the earth is tilted on its axis at the 23.5 degree angle and the seasonal designations here are for the northern hemisphere so we can see the situation in the northern hemisphere and why it's warmer in northern hemispheric summer. We can see here that when it's summer in the northern hemisphere the northern hemisphere is tilted toward the sun so the sun's light is coming in and striking the surface more directly and so the heating power of the sun is higher in the northern hemisphere during summer. Meanwhile in the southern hemisphere that hemisphere is tilted away from the sun and the light gets spread out over a larger area the sunlight is coming in at a lower angle and so the heating power of the sun is reduced in the southern hemisphere that's why the southern hemisphere is having its winter while the northern hemisphere has its summer. We can check out the situation which is just the opposite then in northern hemispheric winter. During the winter's northern hemisphere the winter, the sun angle is low in the northern hemisphere. It's coming in at a lower angle and the light is spreading out over larger areas so the heating power of the sun is reduced because the northern hemisphere is tilted away from the sun and the light is coming in at a low angle. Meanwhile in the southern hemisphere the sun is coming in more directly, the light is coming in more directly and the heating power of the sun is stronger in the southern hemisphere and that's why when it's winter in the northern hemisphere it's summer and warmer in the southern hemisphere. With this basic understanding then we can ask ourselves a couple of interesting questions like what would happen if we change the angle of the Earth's tilt to say 50 degrees so it's much more tilted. Well that would actually result in more extreme seasonal changes because say we had a 50 degree angle of tilt here the northern hemisphere during winter would be tilted even farther away from the sun and the sun angle would be even lower and the sunlight would be spread over a much larger area so the heating power of the sun would be reduced even further making winter in the northern hemisphere even colder. Meanwhile in the southern hemisphere the sunlight would be coming in even more directly and it would be even hotter so you'd have more extreme seasons if you increased the angle of the Earth's tilt. But if you reduced it all the way to zero say the Earth had no tilt on its axis then seasonal variation would go away completely because the angle that the sun's light was coming in to the Earth would not change throughout the year and so we wouldn't have any changes to our seasons.